I'd also add that the much bigger deal (at least if you ever plan to use a flash) is Sync speed. This is much slower than the max shutter speed, but is actually the fastest the shutters can move. Beyond the sync speed, the second shutter is firing before the first finishes opening, so you can't use a single flash pulse for exposing the whole image once you are faster than the sync. A good flash can compensate with high speed sync, but this involves doing multiple flashes and reduces the total power the flash can put out significantly. That can matter a lot doing daytime fills.
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AJ HendersonOct 24 '13 at 13:41

@AJHenderson The original question referenced Canon, which leads me to believe he is comparing the 5D3 with the 6D. By far the most significant difference between those two models is the focus system of each. The difference between 1/180 sec and 1/200 sec is trivial.
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Michael ClarkOct 24 '13 at 23:58

@MichaelClark - ah, yeah, that would be insignificant, though if there was a significant difference, it would be more worthwhile than the max shutter speed. Agree that the most significant difference between the 6D and the 5DIII is the focus system (and depending on use, possibly the additional weather sealing).
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AJ HendersonOct 25 '13 at 1:59

2 Answers
2

In most scenarios the extra stop between 1/4000 and 1/8000 second will make very little difference in terms of freezing motion. 1/4000 will freeze all but the fastest objects you are likely to encounter, and even 1/2000 will freeze world class human athletes and most animals at typical shooting distances.

Where the extra stop will come in handy is when you are in very bright light, have already adjusted your camera to the lowest available ISO and want to use a wider aperture to reduce the Depth of Field (DoF). If you find yourself shooting in such situations often, you will probably wind up eventually acquiring and learning to use Neutral Density (ND) Filters. These reduce the amount of light entering the camera without adding a color cast (hopefully) in order to enable slower shutter speeds and/or wider apertures when desired. Once you start shooting with ND filters the difference between the two camera's fastest shutter speeds will not mean much.

Having said that, there are often other features that differ between such models. In the case of the Canon 5D mark III versus the Canon 6D, for example, the 1-series focus system of the 5D3 is worth the difference in price compared to the less capable 'prosumer' focus system in the 6D, but only if you need the faster, more accurate, and more consistent focus system. On the other hand, the 6D includes built-in WiFi and GPS. If you need those extras, it will cost quite a bit to add them to the 5D3 via external modules.

1/8000 does have it's uses - mostly super-fast sport at close range, but even formula 1 race cars (photographed from a safe distance without a super-telephoto lens) don't show much motion blur at 1/500.

Unless you photograph sports from up close both 1/4000 and 1/8000 are faster than you'll need, my guess is that the two models you are considering may have some differences that you should care about but the max shutter speed isn't it.